1. Technical Field
The present invention relates to an electric power assist steering gear for a vehicle and particularly relates to a center take-off rack and pinion power assist steering gear.
2. Description of the Prior Art
Center take-off rack and pinion hydraulic power assist steering gears are known. Typically, such a steering gear includes an elongate member which moves axially. The elongate member has a rack portion which engages teeth of a pinion gear. The pinion gear is rotated by a steering input shaft. Upon rotation of the input shaft, a directional control valve is actuated. The valve modulates, in accordance with steering input torque, pressurized fluid flow to a hydraulic motor which applies an assist force to the elongate member to axially move the elongate member. The hydraulic motor includes a piston located in a fluid chamber and connected to the elongate member. A linkage is connected to a central portion of the elongate member. The linkage effects turning of the vehicle steerable wheels upon axial movement of the elongate member. The rack and pinion gear set act as a servo mechanism providing feedback to the valve for aiding in control of flow to the hydraulic motor.
The known center take-off hydraulic power assist rack and pinion steering gear commonly have the piston connected to the end of the elongate member. The surface area of the piston against which the fluid pressure acts to move the piston in one direction is different than the area against which the fluid pressure acts to move the piston in the opposite direction. The fluid pressure acts on the entire area of the piston when moving the piston in one direction, whereas when moving the piston in the opposite direction, the fluid pressure acts on the area of the piston minus the area of the elongate member where it attaches to the piston. This has resulted in complications in the design of center take-off hydraulic power assist rack and pinion steering gear.
Electric assist rack and pinion steering gear is also known. One electric assist rack and pinion steering gear is disclosed in U.S. Pat. No. 4,415,054. The steering gear of U.S. Pat. No. 4,415,054 includes a movable elongate member having a rack portion and a screw thread portion. A manually rotatable pinion engages the rack portion of the elongate member. An electric motor encircles the screw thread portion of the elongate member and rotates a ball nut which also encircles the screw thread portion. The ball nut applies an axial force to the screw thread portion of the elongate member to effect axial movement of the elongate member in response to actuation of the electric motor.